The person in Yahoo Help said to post here. Please make it possible to set Incoming Mail to a blacker font; and, larger than 16 point type.
Please make it possible to set ALL Incoming Mail larger than 16 point type.

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C. R. commented
you can control the text size and font choices with your browser, but some will set ALL sites to those choices. In Firefox, unless they changed it, you can pick your zoom text level and it will remember it for that site and not affect other sites. You can set your whole browser font/text size setting to an easier to read size too, in 'one' size increments (14,15,16 etc), and many different font choices, but that will change it for all sites. I have my FF set at 17 and Verdana, but to 'allow sites to use their own font style choices', but you can override that with your own settings too. These are usually under an Options menu. Not sure what Chrome or the new Edge browsers offer, but that's your choice for now, as yahoo doesn't offer font/size choices unless you're in composition mode. And there is no 'blacker' text, it's as black as it can be, but in FF you can choose your own text colors to override a site's colors, for the most part. Learn how to use your browser, it helps.
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V L commented
Please include at least one Serif font to choose from such as Bakerville. A Sans Serif font that is easy to read is Verdana. Believe it or not, Comic Sans is good for dyslexia.